Resistance to bacterial pathogensis often developmentally regulated. In rice, the gene Xa21 controls resistance to a bacterial pathogen with resistance expressed at the adult stage but not at the juvenile stage. Xa21 encodes a receptor-likekinase with a leucine rich repeat (LRR). Levels of Xa21 mRNA do not increase concomitantly with the acquisition of disease resistance Thus the developmental control of Xa21-mediated resistance may be regulated at some post-transcriptional level. Moreover, it is not know which additional gene products contribute to developmental regulation of this resistance. In order to develop strategies for preventing disease during the susceptible stages in development, it is crucial to understand the level at which developmental controlis exerted. The objective of this proposals to determine the post-transcriptional processes that regulate the function of Xa21 and which gene products and activities in the signal transduction leading from Xa21 to a resistance response are developmentally controlled. The central hypothesis guiding the proposed research is that the Xa21-mediated resistance depends on activities that occur post-transcriptionally to Xa21 expression, either at XA21 itself or involving developmentally controlled expression and function of other gene products in the host. The specific aims of the proposal will test predictions based on the central hypothesis: 1.) Activity of Xa21 is controlled post- transcriptionally. We will determine whether levels of XA21 protein and phosphorylation activity parallels resistance in response to infection at juvenile and adult stages. 2.) Coordinately expressed or repressed proteins in host are factors in the signal transduction leading to a resistance response. Study of proteins by 2-dimensional gel electrophoresis and mass spectrometry that are expressed or repressed coordinately with the transition from susceptible juvenile stage to resistant adult stage will allow identification of factors that control or provide competence for Xa21-mediated resistance. 3.) The coordinately expressed calcium binding protein FIERG1 is active at the onset of resistance and shifting its expression will shift the onset of resistance. FIERG1-silenced and overexpression lines will allow assessment of its role in control of or competence for developmentally controlled resistance. The results obtained from these studies will lay the ground work for design of strategies for the establishment of Xa21-specific resistance throughout the life of the plant.